1. Field of the Invention
This invention relates to hydraulically driven machinery and more particularly it concerns novel arrangements for monitoring and controlling the movement of hydraulically driven injection rams.
2. Description of the Prior Art
The present invention is particularly suitable for use with die casting machines. In these machines a fluid material to be cast, e.g. molten aluminum, is loaded, e.g. by ladling, into a shot sleeve which is connected to a die casting cavity. A hydraulically driven ram then moves rapidly through the shot sleeve to force the molten material into the die cavity. The molten material is chilled by contact with the cavity surfaces and hardens. Thereafter the cavity is opened and the hardened material is removed.
It is important in die casting operations to control the rate of material injection into the mold with a high degree of precision. If the injection speed is too great the material may wear the cavity surfaces and may produce bubbles in the finished castings. If the injection speed is too slow the material will cool improperly in the mold and will result in unacceptable castings. It is also advantageous to vary the injection rate as the cavity filling operation proceeds. This variation in injection rate will correspond to the particular size and shape of the cavity being filled; and accordingly it is desirable to provide a high degree of flexibility in the injection operation.
A number of techniques have been employed in the past to monitor and control the speed of injection of material into a mold cavity. U.S. Pat. Nos. 3,726,334 and 3,729,047 show arrangements for monitoring injection ram velocity and position. U.S. Pat. No. 3,726,334 suggests the provision of magnetic or mechanical elements along the length of the ram and a magnetic or optical sensing means fixed along the path of movement along the elements. As each element passes the sensing means it causes an electrical pulse to be generated. Ram position can be ascertained from the number of pulses which have occurred and ram or injection velocity can be ascertained from the rate at which the pulses occur. In U.S. Pat. No. 3,729,047 a plurality of glass filaments are arranged in a line along the path of ram movement and rays from a light source pass through the filaments. A light reflector is mounted to move with the ram and to pass by the ends of the filaments. As the reflector passes by each filament it redirects light through another filament to a photocell which generates an electrical pulse. Again, the number of pulses corresponds to ram position and the rate of occurrence of pulses corresponds to ram velocity.
U.S. Pat. Nos. 3,767,339, 3,893,792 and 4,066,189 show hydraulically driven injection molding devices with pressure sensing arrangements to control ram movements. U.S. Pat. No. 3,767,339 also shows a rack and pinion tranducer arrangement for monitoring the ram movements.
The position and velocity sensing devices of the prior art have not been entirely satisfactory for injection molding operations. This is because the ambient conditions in the vicinity of the ram are not conducive to precision measurement. More particularly, the material being molded often spills and contaminates the sensing or signal generating elements. Also, the dust, dirt and gases which accompany the feeding of molten material into the shot cylinder may interfere with the reliable operation of the sensing and signal generating elements.